Method and apparatus for determining rotational speed

Abstract

A method and apparatus are disclosed for providing improved rotational speed determination for a rotating shaft. Two probes positioned adjacent a circular disk which rotates with the shaft scan markings on the disk and generate four out-of-phase signals in response to rotation of the shaft. A switch, controlled by logic circuitry which detects intersections of the signals, develops a composite saw-tooth-shaped signal from the four signals. A counter counts the number of crossings of the composite signal with a reference "zero" value to develop a determination of the number of discrete angular positions traversed by the shaft during a specified time interval. A memory stores the value of the composite signal at the start of the time interval. Computation circuitry develops a rotational speed based on the counted number of positions, corrected by a factor derived from the difference between the stored value and the value of the composite signal at the end of the interval. Time control circuitry prevents the start or end of the time interval during switching and prevents switching during the start or end of the time interval.

What is claimed is: 
     
       1. In an apparatus for determining the rotational speed of a rotating member having a digital tachometer signal generator which produces an angular signal that varies steadily at least section-wise and approximately periodically with the angle of rotation of the rotating member, the respective cycle length of said signal being defined by angular regions traversed by the rotating member between approximately equidistant discrete angular positions defined by the intersection of the angular signal with a reference value, and the instantaneous amplitude of said signal corresponding to the respective instantaneous angle of rotation relative to an adjacent one of the discrete angular positions, and which delivers a counting pulse whenever the instantaneous value of the angular signal is equal to the reference value; and   a counter which counts the number of count pulses delivered within the measurement period for determining the number of said traversed angular regions, the improvement comprising:   an output terminal on said signal generator for delivering the angular signal from the signal generator;   a memory connected to the output terminal to receive and store until the end of the measurement period, in response to an initializing pulse, the amplitude of the angular signal corresponding to the instantaneous angle of rotation of the member at the beginning of the measurement period; and   computation circuitry connected to receive, in response to an end pulse at the end of the measurement period, the amplitude of the angular signal corresponding to the instantaneous angle of rotation of the member at the end of the measurement period and the stored amplitude from the memory, to define a correction factor corresponding to the difference in the instantaneous angles of rotation at the beginning and end of the measurement period, and to compute a corrected total angular displacement based on the counted member of traversed angular regions and the defined correction factor.   
     
     
       2. Apparatus as defined in claim 1, wherein the digital tachometer signal generator produces at least two mutually phased-shifted signal curves; and further comprising a direction discriminator connected between the signal generator and the counter and cooperating with the counter so that upon reversal of the direction of rotation the angular regions traversed in reverse direction are counted with opposite sign. 
     
     
       3. Apparatus as defined by claim 1, wherein the digital tachometer signal generator produces four signals, phase-shifted relative to each other and all having sinusoidal, triangular or trapezoidal wave shapes; and further comprises a switch which serves to superpose the approximately linear sections of the signals to form a composite output angular signal having an approximately sawtooth wave shape. 
     
     
       4. Apparatus as defined by claim 3, further comprising a time control unit responsive to the receipt of start and stop electrical time pulses for fixing the length of the measurement time period, and for delivering the initializing and end pulses to other portions of the apparatus. 
     
     
       5. Apparatus as defined in claim 4, wherein the time control unit further comprises means for delaying the initializing or end pulse until switching is completed whenever such a pulse would otherwise be delivered during switching. 
     
     
       6. Apparatus as defined by claim 4, further comprising a timer driven by the initializing and the end pulses for determining the length of the measurement period; and the computation circuitry further comprising a divider connected to form the quotient of the corrected total angular displacement signal and the length of the measurement period determined by the timer. 
     
     
       7. Apparatus as defined in claim 6, further comprising a second memory responsive to actuation of the switch, to store the maximum amplitudes prior to each switching of the composite sawtooth-shaped signals; and wherein the computation circuitry further comprises means connected to the second memory to determine mean angular distance between discrete angular positions and means for normalizing the correction factor for the mean angular position. 
     
     
       8. Apparatus for determining the rotational speed of a rotating member, comprising a digital tachometer device having: (a) a signal generator for generating an electrical signal system consisting of at least two mutually phase shifted and steadily varying angular signals of approximately sinusiodal, triangular or trapeziodal wave shapes periodically in response to the rotation of the member through successive approximately equidistant discrete angular positions;   (b) means for connection between said signal generator and a counter, said means for connection comprising a direction discriminator;   (c) said counter, cooperating with the direction discriminator for counting the number of occurrences of a particular signal event within a specified time interval, to develop a digital determination of the rotational speed corresponding to the number of the discrete angular positions traversed during the time interval, so that upon reversal of the direction of rotation the particular signal events are counted with opposite sign;   (d) an output terminal on said means for connection for delivering the angular signals from the signal generator to a switching means, said switching means superposing the approximately linear sections of the steadily varying signals to form a composite output angular signal having an approximately saw-tooth wave shape and a period length equal to the time distance between said particular signal events;   (e) a memory connected to said switching means to receive and store until the end of the measurement period, in response to an initializing pulse, the instantaneous amplitude of the composite angular signal corresponding to the instantaneous angle of rotation of the member at the beginning of the measurement period; and   (f) computation circuitry connected to receive, in response to an end pulse at the end of the measurement period, the instantaneous amplitude of the composite angular signal corresponding to the instantaneous angle of rotation of the member at the end of the measurement period and the stored amplitude from the memory, to define a correction factor corresponding to the difference in the instantaneous angles of rotation at the beginning and end of the measurement period, and to compute a corrected total angular displacement based on the counted member of tranversed angular regions and the defined correction factor.   
     
     
       9. Apparatus as defined by claim 1, wherein the electrical signal system generated by the digital tachometer device comprises a first angular signal and a 90°-phase shifted second angular signal and the composite angular signal is cyclically composed by a section of the first, the second, the inverted first and the inverted second angular signal.